Nathan Fillion is in for some “sweet, sweet revenge,” as his former Firefly costar Adam Baldwin calls it.

“It was great to be able to turn the tables on him finally. I was playing the subservient Jayne Cobb to his Mal Reynolds for a while and to get a chance to be a character who was a bit over his rank and be able to push him around a bit was just liberating,” he says. “Plus, the opportunity to work with Nathan again — he’s such a loving and giving actor for anybody that he works with and especially for the old [Firefly] crew.”

On tonight’s Castle, Fillion’s Richard Castle meets Baldwin’s Detective Nathan Slaughter, a no-nonsense badge from the gang unit who takes the writer along for a few adventures, acting as a pseudo-muse in place of Beckett. (She and Castle are still on the outs.) But Beckett he is not. “[Slaughter] uses unorthodox methodology to get results,” teases Baldwin. (See a preview below.) “When he’s given the opportunity to bring Castle along and show him the ropes — for selfish reasons, of course — Castle finds it a bit repulsive but also a bit attractive. There’s that thrilling heroics that he gets to be a part of.”

And when the script called for such heroics, Baldwin dove right in — from stunt driving to gun wielding. But that’s not unexpected. Following his Firefly days, Baldwin spent half a decade playing tough guy John Casey on Chuck, which closed out its run in January with a polarizing and open-ended series finale. (Baldwin stands behind the ending, saying, “Life is like that. The only real finality there is death, and we didn’t have that. I liked that open-endedness.”) And while Fillion never got a chance to be a part of the Chuck world, Baldwin revels in the chance to join the Castle ecosystem — should a chance to return ever appear, he’d “certainly accept it happily.”

But should that chance not arise for a while, Baldwin says they made sure to throw a few Firefly nods into tonight’s episode for loyal Browncoats. “I think that Firefly fans will not be disappointed. We don’t hit it over the head too much, but we maybe used a two-by-four. Nothing more serious than that,” he says with a laugh.